Dielectric gases are used in various electrical apparatuses such as, for example: transformers, electric cables or buses, and circuit breakers or switchgear. For example, see U.S. Pat. No. 7,807,074 (Luly et al.). In such electrical devices, dielectric gases are often used in place of air as an electrical insulator due to their higher dielectric strength (DS). Such dielectric gases allow higher power densities as compared to air-filled electrical devices.
Most significantly, sulfur hexafluoride (SF6) has become the dominant captive dielectric gas in many electrical applications. SF6 is advantageously nontoxic, non-flammable, easy to handle, has a useful operating temperature range, and excellent dielectric and arc-interrupting properties. Within transformers, it also acts as a coolant. Blowers within the transformer often circulate the gas aiding in heat transfer from the windings.
However, a concern with SF6 is its 3200 year atmospheric lifetime and global warming potential (GWP) of about 22,200 times the global warming potential of carbon dioxide. At the December 1997 Kyoto Summit in Japan, representatives from 160 countries drafted an agreement containing limits for greenhouse gas emissions. The agreement covers six gases, including SF6, and included a commitment to lower the total emissions of these gases by the year 2010 to levels 5.2% below their total emissions in 1990. See UNEP (United Nations Environment Programme), Kyoto Protocol to the United Nations Framework Convention on Climate Change, Nairobi, Kenya, 1997.
Certain perfluorinated nitriles CF3CN, C2F5CN and CF3CF2CF2CN have been disclosed for use as gaseous dielectric materials in U.S. Pat. No. 3,048,648 (the '648 patent). However, the toxicity of these nitriles is higher than would be considered acceptable for use as a gaseous dielectric material. In addition, the '648 patent describes the nitriles as “more particularly a member of the group of perfluoro-n-alkylnitriles”. Efforts have been made to reduce the toxicity of CF3CF2CF2CN with the addition of nitrite esters (see U.S. Pat. No. 4,547,316).
The National Institute of Standards and Technology (NIST) has published Technical Note 1425: “Gases for electrical Insulation and Arc Interruption: Possible Present and Future Alternatives to Pure SF6”, which identifies, as possible replacements, mixtures of SF6 with either nitrogen or helium, or high-pressure nitrogen. Some other replacement mixtures suffer from release of free carbon during arcing, increased toxicity during or after arcing, and increased difficulty in gas handling during storage, recovery and recycling. Also identified are perfluorocarbon (PFC) gases that might also be mixed with nitrogen or helium, like SF6. However, PFCs also have high GWPs so the possible reduction in environmental impact of such strategies is limited.